1. Field of the Invention
The present invention relates to a method for suppressing quantization noises which are superimposed on an audible tone having a sinusoidal waveform upon transmission thereof in a digital telecommunications exchange system.
2. Description of the Prior Art
Upon transmission of speech signals in digital exchange systems which are based, for example, on a sampling frequency of 8 kHz, coded samples are transmitted at time intervals of, respectively, 125,.mu.sec. Since the sampling frequency is not a whole multiple for the frequency of the audible tones normally employed, quantization noises arise, these being particularly attributable to the common subharmonics of the audible frequency and the sampling frequency appearing with relatively high amplitudes. These common subharmonics, as well as their upper harmonic waves, are distributed at short intervals over the entire voice band and can therefore not be canceled by way of filters. Such quantization noises become noticeable in a particularly disturbing manner with signals having a sinusoidal waveform as represented by audible tones. Avoiding such disruptions, in that only audible tones whose frequency is a subharmonic of the sampling frequency are generated, can only be poorly achieved in view of the signaling requirements to be met, and added thereto is the fact that the generation of the audible tones advantageously occurs with the assistance of digital tone generators which emit amplitude values stored in coded form as a pulse code modulation (PCM) word with a cyclic repetition, the plurality of the amplitude values being a power of two for economic reasons.
The frequency of the tones thereby generated is determined by jumping a suitable plurality of stored PCM words. Derived therefrom, however, is that any desired frequencies and any desired subharmonics of the sampling frequency cannot be generated with such tone generators.